Radon 222 and its isotope Radon 220 (Thoron) are radioactive, chemically inert gasses which occur in the natural radioactive decay chains of Uranium and Thorium. As gasses they are able to diffuse out of the soil or concrete where they are formed and collect in spaces where people live and work.
The progeny of Radon are metals such as Polonium, Bismuth and Lead which are also radioactive. When breathed in, they tend to stick in the throat or lungs. Subsequent radioactive decays occur on the tissue surface and bombards the tissue with high-energy alpha and beta particles, causing damage to the cells. The radiation damage caused by these radon progeny can result in cancer.
When a Radon atom decays, it emits an alpha particle with an energy of several million electron volts. The alpha particle ionizes a path in the air several centimeters long, creating several hundred thousand pairs of positive atoms and negative electrons. The Polonium daughter of the Radon recoils from the decay and itself becomes positively ionized. Within a millisecond, in normal humidity, those positive atoms and negative electrons which have not recombined with each other will have acquired several water molecules to form a cluster molecule around the single charge. These charged cluster molecules are stable and have a lifetime in clean, free air of several minutes. Atmospheric electricians refer to such cluster molecules as "Fast Ions". The cluster molecule containing the radioactive Polonium atom is called an "unattached daughter".
The fast ions are responsive to electric fields, including those created between the ion and its image in a surface or in a large particle. They are thus sticky and are quickly swept out of the air if there are particles suspended in the air or if the walls are close. When a fast ion is captured by a dust particle or "Aitken Nucleus", it becomes a "slow ion" as it is no longer mobile in an electric field. If the cluster molecule around the Polonium daughter is captured by a heavy particle it becomes "attached".
Most radiation-damaged cells either die or are repaired satisfactorily. A very few, however, may be damaged in such a way as to promote unlimited reproduction causing cancer. Lung tissue is most sensitive to is kind of damage. It is recognized that exposure to radiation from Radon and its progeny is the second biggest cause of lung cancer, after smoking. It is also the biggest cause of death from the built environment.
Typical results show radon levels in outside air over land of 1 to 50 Bq/m.sup.3 and indoors anything from outside levels to 1,000 Bq/m.sup.3 or more. Extended exposure to levels in excess of 200 Bq/m.sup.3 (UK and Hong Kong) or 150 Bq/m.sup.3 (USA) are considered dangerous and require mitigation.
Most Radon-induced lung cancer occurs in the bronchial region rather than deeper in the lungs. This is because the unattached radon progeny are readily deposited on the bronchial wall whereas the heavier, attached daughters are carried on their particles by the air flow deeper into the lungs where mechanisms exist to remove them. Thus it is that unattached daughters have more than fifty times the deposition rate of attached daughters and therefore more than fifty times the radiation dose efficiency, as noted in Radon versus Rn Daughters, Hans Vanmarcke and Paul Berkvens, Health Physics, vol. 56 #2, pp. 229-231, 1989.
Because of the variation of deposition rate between unattached and attached progeny, it transpires that the "working level", or the total concentration of Radon daughters is not an accurate measure of the health risk in a given environment and that the Radon concentration is probably a more useful indicator. This is one reason why the United States Environmental Protection Agency expresses its recommendations in terms of Radon concentrations rather than of concentrations of Radon daughters.
The two major techniques for measuring Radon are:
a) passive, by collection on some absorbing material such as activated carbon, and subsequent analysis, and PA1 b) active, by some sensor and electronic device such as the NITON RAD7 (trademark) electronic Radon detector.
These methods measure just the concentration of the Radon gas and not the daughters.
Instruments are available for measuring the "Working Level" by determining the total concentrations of Radon Progeny. Typically, air is drawn through a filter and the alpha radiation from the filter is measured. Some of the more sophisticated instruments will analyze the spectrum of the alpha energy thus determining the distribution of different daughters on the filter, giving additional information including, for instance, the contribution to the radioactivity from Thoron daughters. These instruments do not distinguish between the attached and unattached fractions of the progeny, collecting both indiscriminately.
A prior art method of measuring the unattached fraction involves collecting the progeny in such a way that attached daughters escape collection, leaving only the previously unattached daughters, and then measuring the alpha radiation from the collected progeny. The standard technique is to use a screen with a mesh big enough to allow nearly all the heavier particles to pass through unimpeded while collecting the small, more mobile unattached daughters on the screen as described in Unattached fraction of short-lived Rn decay products in indoor and outdoor environments: an improved singe-screen method and results, A. Reineking and J. Porstendorfer, Health Physics, Vol. 58 #6, pp. 715-727, 1990. Other methods have taken advantage of the positive charge on the majority of the unattached daughters to drive them by means of an electric field to a collecting plate or screen where their radioactivity is measured, but this technique is not popular because the uncharged daughters are not collected as discussed in An evaluation of unattached Radon (and Thoron) daughter measurement techniques, A. W. Van der Vooren, A. Busigin and C. R. Phillips, Health Physics, Vol. 42, pp. 801-808, June 1982. A combination of the screen or `diffusion battery` to collect the unattached progeny, and a filter to collect the rest provides a means to collect and measure both attached and unattached progeny fractions at once as disclosed in U.S. Pat. No. 4,847,503 to Tetley, W. C. et al., Jul. 11, 1986.